I studied the spatial associations and distribution of mound-building mice Mus spicilegus Petenyi during the breeding periodin northern Bulgaria. The most frequent spatial association observed was between a single male and a female. Other associations between two or three males, and between a male and two or three females were also observed. Female social groups were seldom observed. Home ranges of females were exclusive. Male home ranges were larger than those of females. Home ranges of about half of the males overlapped home ranges of one female; the remaining males had home ranges that overlapped home ranges of two adult females. These results suggest that M. spicilegus has a variable socioreproductive mating system that is either monogamous or polygynous. The predominance of male-female pairs and solitary females, as well as the exclusive home ranges of females, suggests that monogamy is the preferred mating system[Acta Zoologica Sinica 53(1): 22 –28, 2006].